Method of pulping wood



and usually ninety-five per cent;

Patented Aug. 7 1928. p

UNITED STATES v 1,679,682 PATENT OFFICE.

I FRANCIS G. BAWLING, OI KNOXVILLE, TENNESSEE.

mrrnon or runrme woon.

No Drawing. Application flied July 21,

This invention relates to an im rovement in the method of pulping wood w ereby the total yield of pul from a given amount of wood is increase beyond that theretofore obtainable.

In the acid and alkaline pulping processes known as the so-called soda and sulphite processes, the yield of pulp does not greatly exceed fifty per cent by weight of the Wood and usually of pulp on t 1e weight of wood are considered ood. The amount of cellulose (as determined by the Cross and Bevan method of analysis) present in wood varies from fiftyfive to sixty-five per cent. Pulps prepared by the acid and alkaline pulping processes consist of at least ninety per cent cellulose,

The difference between the amount of cellulose present in a definite weight of wood, and the cellulose present in the pulp obtained from this weight of wood represents that part of the cellulose which is attacked and dissolved by the acid and alkaline pulping chemicals used in the pulping processes. method of preventing the solution of at least art oi. this cellulose would increase materially' the yields of ulp consisting essentially of cellulose o ta nable from wood. The present invention 1s directed toward providing a satisfactory way to solve this problem.

In the preparation of pulp by the treatment of wood with a ueous solutions of neutral or slightly alka ine chemical reagents, in which such treatments are carrled out at temperatures of at least 100 C. and not above 150 C., it has been discovered that the pulps thus obtained contain all the cellulose present in the original wood. The neutral or slightly alkaline chemical reagents have had practically no solvent effect on the cellulose. A combination of chemicals in aqueous solution such as sodium sulphite and the sodium salt of a weakly ionized poly-basic acid, such as sodium carbonat'or bicarbonate may be used for this particular chemical treatment of wood. Pulps prepared by such a mild chemical treatment of the wood contain in addition to all the cellulose present in the original wood, about sixty per cent of the lignin contained in the original wood. In order to produce a pulp consisting substantially of cellulose, a further chemical treatment is necessary. This is given by subjecting the chemically ields of forty percent by weight\ 1926. Serial N0. @104.

treated wood to the action of a non-oxidizing chemical which is capable of dissolving lignm, as for instance, either sulphur dioxide or a solution of a strong alkali such as .caustic soda or sodium sulphide. The first chemical treatment of the wood renders the cellulose more stable to the action of such acids and alkalis, which are to be used in the second chemical treatment for the purpose of removing or dissolving the lignin.

As an example of the process, the first chemical treatment may be made by steaming chips of wood such as aspen, then heating them with an aqueous solution, containing forty grams per litre of sodium sulphite and fifteen grams per litre of sodium bicarbonate. The chips may be steamed in a digester, and at the end of the period of steaming the chemical solution may be added to the hot chips. The digester is closed and its contents are heated to a temperature of 125 C. or thcreabouts and maintained at this temperature for an hour or so until the contents of the digester are uniforml heated throughout their mass. The chemical liquor is now withdrawn and the second chemical treatment begun by the introduction of sulphur dioxide gas into the digester. At first the gas is absorbed by the small amount of chemical remaining in the wood from the first chemical treatment. The presence of sulphur dioxide causes an increase of gauge pressure, above that required by steam at the temperature of the digester. The gauge pressure may thus be increased by twenty or thirty pounds 'and the temperature may be increased to a point not in excess of 150 0., and held there until the wood is cooked to the required degree of purity as regards its cellulose content.

The second chemical treatment in which sulphur dioxide is used may be replaced by one in which a strong alkaliis employed, such as an aqueous solution of caustic soda or sodium sulphide or a mixture of the two. The temperature required for this alkaline chemical treatment will not be in excess of 180 G.

Since the wood is already partially dissolved in the first chemical treatment to the extent of ten to twenty per cent of the weight of the wood, the second chemical treatment may be made by the use of dilute alkaline solutions, such as an aqueous solution of ten grams per litre of caustic soda or of sodium sulphide. The second chemical treatment is ended when the pulp consists mainly of. cellulose, the time thus taken may be as much as twelve hours.

In both these examples the. first chemical treatment dissolves a certain amount of the wood, usually about twenty per cent of its weight, the soluble portion consistin of a part of the lignin, some organic aci s and other compounds of undetermined constitution, but leaves the cellulose undissolved. The second chemical treatment removes the greater part of the lignin from the treated wood while leaving the cellulose as a residue in the form of pulp. The second chemical treatment does not employ acid or alkaline solutions of so great a concentration as are commonly employed for the pulping of wood by the so-called acid and alkaline pulping processes.

The use of non-oxidizing chemicals for the removal of lignin in the second chemical treatment permits the use of digesting vessels which are incommon use in the industry at the present time. Oxidizing solvents for lignin would require the use of special equipment, and such chemicals would have a tendency to oxidize and dissolve the cellulose in the pulp, and so decrease the 2. The process of treating wood fibrous material to convert it into pulp which process includes treating wood chips with an aqueous solution of sodium sulphite and the sodium salt of a weekly ionized polybasic acid at a temperature not lower than 100 C. and not above 150 O.,,removing the unabsorbed chemical liquor from the wood thus treated, and subsequently treating the wood residue with a non-oxidizing chemical capable of dissolving Ii '11 and at a temperature. not below 100 and not above 3. The process of treatin wood fibrous material to convert it'into pu p, which proc- 4. The process of treating wood fibrous material to convert it into pulp which includes treating wood chips with an aqueous solution of sodium sulphite and the sodium salt of a weakly ionized polybasic acid at a temperature not lower than 100 C. and not above 150 0., removing the unabsorbed chemical liquor from the wood thus treated, and subsequently treatingthe wood residue with a non-oxidizing chemical capable of dissolving lignin.

5. The process of treating wood fibrous material to convert it into pulp which rocess includes treating wood chips wit an aqueous solution of sodium sulphite and the sodium salt of a weakly ionized polybasic acid at a temperature not, lower than 100 C. and not above 150 (1, removing the unabsorbed chemical liquor from the wood thus treated, and subsequently treating the wood residue with a non-oxidizing chemical capable of dissolving lignin.

6. The process of treating vegetable fibrous material to convert it into pul which process includes treating chips wit an aqueous solution of.sodium sulphite and thesodium salt of a weakly ionized polybasic acid at a temperature not lower than 100 C. and not above 150 0., removing the unabsorbed chemical liquor from the chips thus treated, and subsequently treating at a higher temperature the chips residue with a non-oxidizing chemical capable of dissolving lignin.

In testimony whereof I aflix my signature.

FRANCIS e. RAWLING. 

